The invention relates to an antenna element with coplanar waveguide for wireless communications.
In the field of car-to-car communication, specific antenna elements are provided for wireless communication between cars equipped with enabled on-board units. On-board units may be configured to detect information regarding current traffic situations (e.g. traffic jam, icy road, construction works) as well as car specific parameters (e.g. velocity, moving direction, acceleration, outside temperature, windscreen-wipers on).
This information can subsequently be transmitted via an air interface to other cars located in the same geographical region and equipped with accordingly enabled on-board units. A receiver of an on-board unit may thereafter analyze the information from various cars in order to improve the traffic safety as well as the efficiency for each car individually. Accordingly, the design of antenna elements has to meet technical challenges that are particularly present in the field of car-to-car communication.
One technical challenge in the field of car-to-car communication relates the directional radiation pattern of the antenna element. Specifically, it is advantageous for the antenna element to provide for an omni-directional radiation pattern in the horizontal plane.
The requirement for an omni-directional radiation pattern in the horizontal plane is inherent to the utilization of the antenna element for car-to-car communication. In combination with a car, the antenna element is to be used for wireless communication with other cars that can be positioned at any direction with respect to the car. Accordingly, it would be disadvantageous if the antenna element would realize a directional and not the required omni-directional radiation pattern in the horizontal plane.
In the context of this description, the term omni-directional radiation pattern of an antenna element is to be understood as its capability to radiate equal power in all directions perpendicular to the extent of the antenna element, i.e. in the horizontal plane.
Another technical challenge in the field of car-to-car communication relates to the dimensions and the shape of the antenna element for it to be incorporated in existent roof-top antenna assemblies.
The requirement for suitable dimensions and shape of the antenna element becomes immediately apparent from the necessity to incorporate the antenna element in an existing roof-top antenna assembly. Roof-top antenna assemblies have developed in recent years allowing various antenna elements to have a mounting position on the roof-top on the car. At the same time the roof-top antenna assembly provides a protective cover against environmental influences, for instance, moist climate and wind. Accordingly, it is advantages for antenna elements to be incorporated into the roof-top antenna assembly.
In recent years, roof-top antenna assemblies have been subject to frequent re-designs in order to incorporate antenna elements, for instance, for analog and digital radio reception, for GPS reception, for GSM/3G/4G communications, for WIFI communications and for television reception. Now, for an antenna element for car-to-car communication to be incorporated into an existent roof-top antenna assembly, it is a requirement for it to have dimensions and a shape to still geometrically fit into the roof-top antenna assembly, namely to fit in addition to various other antenna elements.
In the context of this description, the term car-to-car communication is to be understood as wireless communication in the frequency region of 5.8-6 GHz in Europe and North America. For example, the wavelength λ of a radio wave at the desired frequency of 6 GHz corresponds to: 1·λ=50 mm.
Various designs of antenna elements have been discussed in the past, which are however disadvantages in view of the technical challenges present in the field of car-to-car communication named above. In the following, recent developments for antenna elements are briefly summarized.
U.S. Pat. No. 6,337,666 B1 relates to an antenna element that is printed on opposite sides of a dielectric substrate. An elongated first dipole half element is provided on one side of the dielectric substrate. A second dipole half element is provided on the opposite side of the dielectric substrate. Although the antenna generates an omni-directional pattern at horizon, the construction requires printing on two sides of the dielectric substrate. Specifically, for the second dipole half element to have an effect on the first dipole half element, the dielectric substrate needs to be thin (for example 0.005″ to 0.125″).
U.S. Pat. No. 6,559,809 B1 relates to a two-sided planar antenna configuration. On one side of a printed circuit board, there is provided a conductor including a microstrip feed line portion and a radiating poise portion. The other side includes a ground plane coupled with a structure functioning as a planar waveguide. As already mentioned above, the manufacturing of conductors on two sides of a printed circuit board is complex. Further, the two sides need to be co-located at close proximity, namely a distance of substantially less than one wavelength.
A disadvantageous embodiment is also described where the printed circuit board antenna is provided on a single side with a centre conductor for RF signal transmission and an outer conductor for a corresponding grounding potential. However, this design is described as being less flexible in increasing the impedance seen by the common mode current in the path to the feed line ground plane.
U.S. Pat. No. 7,965,242 B2 (filed as US 2010/0328163 A1) relates to a dual band antenna including a dual-band strip line monopole element. The monopole element includes a radio frequency choke, such as a planar waveguide strip located at one end of the element above a lower portion of the element. The overall length of the monopole element is selected so as to resonate at a first desired frequency. The length of the lower portion is selected so as to resonate at a second desired frequency. The antenna also includes a first reflector element for the first desired frequency and a second reflector element for the second desired frequency.
The dual band antenna is described as advantageous with respect to two spaced-apart frequencies, e.g. 2.4 GHz and 5 GHz. However, the design is disadvantageous with respect to single frequency band for car-to-car communication. Further, the first and second reflector elements prevent the antenna from having an omni-directional radiation pattern.
Zachou, V. et. al.: “Planar Monopole Antenna with Attached Sleeves”; IEEE Antennas and Wireless Propagation Letters, Vol. 5, p. 286-289, 2006 relates to an antenna element that consists of a printed monopole with one or two sleeves connected on each side thereto and fed by a coplanar waveguide line. Switches are used to control the length of the monopole and the sleeves and to tune the resonant frequencies of the antenna. In this design, a first resonance frequency is determined by the length of the monopole whereas a second resonance frequency is determined by the length of the sleeves and their activation.
The single- or dual-sleeved antenna configuration requires conductors, i.e. the sleeves, to be provided on each side of the monopole facing in the direction of the monopole's free end. Accordingly, the design is disadvantages with respect to the dimension and shape.
Dong, T. and Chen Y.-P.: “Novel Design of Ultra-Wideband printed double-sleeve Monopole Antenna”; Progress In Electromagnetics Research Letters, Vol. 9, p. 165-173, 2009 relates to a printed sleeve monopole antenna element. The antenna element is fed by a coplanar waveguide. Double sleeves with different sizes have been added to the ground plane. Thereby, the antenna element has ultra-wideband impedance characteristics.
The printed sleeve monopole antenna element requires conductors, i.e. the sleeves, to be provided on each side of the monopole facing in the direction of the monopole's free end. Accordingly, the design is disadvantageous with respect to the dimension and shape.